The present invention relates to a method for controlling the operation of a compressor, in which the compressor is switched off by a control unit to avoid thermal damage if an estimated temperature value Ts(Tc) calculated by said unit exceeds an upper threshold value Tmax, or remains switched on or is switched on if there is a compression requirement and if a lower threshold value Tmin is not reached.
It is generally known that compressors with which a gaseous or liquid medium can be brought to a pressure above the ambient pressure are used in motor vehicles. This gaseous or liquid medium is often used as a control pressure medium, with which for example actuators such as piston-cylinder arrangements can be acted on directly or via a pressure medium accumulator.
One application in motor vehicles arises from the necessity to supply the pneumatic springs of a level control system with compressed air in such a way that they can move the vehicle to a distance from the surface of the roadway that is appropriate for the driving situation. Since such a level control system does not constantly provide a height adjustment of the vehicle, a compressor belonging to such a system is only put into operation when the need arises according to requirements. Compressors of this type are generally formed as electromotively operated piston compressors.
In the effort to minimize the costs of compressors, small compressors are being increasingly used, with thermal problems possibly occurring if they are operated for longer periods, since their components may heat up to unallowably high levels during lengthy operation. In such cases, the damage generally occurs first at the outlet valve or at the piston seal of a piston compressor, which can ultimately lead to failure of the compressor, and consequently of the level control system.
To avoid operationally induced damage of this kind, there is for example, according to DE 15 03 466 A1, DE 19 43 936 A1 and EP 12 53 321 A2, the possibility of measuring the temperature of the compressor directly in the area of said components and, in the event of thermal overloading, switching the compressor off to cool down.
However, this type of construction entails the disadvantages that the temperature sensors necessary for this purpose are comparatively expensive and can often only be accommodated with difficulty in small compressors on account of the confined installation space in the area of interest. Although EP 12 53 321 A2 indicates that the operation of the compressor can also be controlled without temperature sensors on the basis of a thermal model, the content of such a measuring or control method is not defined.
In addition, it is known from DE 39 19 407 A1 and DE 40 30 475 A1 to determine the thermal loading of such a compressor via the electrical power consumption and/or the operating time of the electric motor belonging to the compressor. Taking a similar direction is the proposal that has become known from DE 43 33 591 A1, that of influencing the control of a compressor by adding up its individual on times and individual off times, which can be used as a measure of many influencing factors for the thermal loading of the compressor.
Another approach is disclosed by DE 198 12 234 C2, according to which a compressor can be variably operated with regard to its on and off times. In this case, the currently applicable on period at a given time is to be adapted to the currently applicable operating conditions of the compressor. Serving as parameters as a function of which the on period of the compressor is varied are the heat transfer conditions prevailing between the compressor and the air surrounding it.
In this case, the on period may be varied for example as a function of the air temperature and air flow rate prevailing in the surroundings of the compressor in such a way that the on period is shortened if the ambient compressor temperature increases and is lengthened if it decreases. The ambient compressor temperature can in this case be determined on the basis of a model calculation from the currently applicable vehicle outside-air temperature and/or the vehicle-engine intake-air temperature. The disadvantage of this method is that, like all on-period methods, it is very inaccurate, because it does not take into account the thermodynamic properties of the compressor itself. Therefore, the control does not for example have any influence on the temperature band in which the compressor is ultimately operated.
Finally, DE 196 21 946 C2 discloses a method for the temperature-assisted control of a compressor for a pneumatic suspension of a motor vehicle which takes the form of an estimating method and manages without a separate temperature sensor on the compressor. For this purpose, it is provided that the compressor is switched off by a control unit if an estimated temperature value calculated by it exceeds an upper threshold value, or is switched on, or allows switching on, if a lower threshold value is not reached. For this purpose, when the compressor is switched on, the last estimated temperature value in each case is increased by a specific temperature increment, the amount of which is dependent on the level of the last estimated value.
In this case, the estimated value is raised by a predetermined positive gradient during compressor operation and lowered by a predetermined negative gradient while the compressor is at a standstill. It is disadvantageous that the linear relationships used as a basis for this method cannot exist as such in reality, since the temperature changes are greater when there are large temperature differences than when there are small temperature differences. Furthermore, the temperature increment does not occur instantaneously in reality, so that control-related availability of the compressor is also disadvantageously lowered in this area.
Against this background, the object of the invention is to present a method by which the currently applicable temperature at a compressor component at risk of being damaged can be estimated more accurately than before, without use of a temperature sensor built into the compressor, so that such a compressor can be operated for longer than previously possible under rising component temperatures.